Non-contacting sensors are well-known Such sensors may include one or more stationary inductance coils and a movable member which moves in the field of the stationary inductance coils in accordance with the parameter being monitored to change the inductance of the stationary coils. Non-contacting sensors are especially useful because they are not subject to wearing as are those sensors in which the moving and stationary parts are in contact. For example, in a simple potentiometer having a wiper blade which moves along a resistance winding, the constant moving, frictional contact between the wiper blade and the resistance winding will cause wear of one or both parts.
One technique for processing the signals of an inductance coil sensor to develop an indication of the parameter being monitored involves measuring the effect of a shift in resonance frequency of a tank circuit which includes, as one of its components, the stationary inductance coil. As the movable member alters the inductance of the stationary inductance coil, a corresponding shift in resonance frequency of the tank circuit is developed.
Such an approach, which may be characterized as time-dependent because frequency is a time-related variable, is to be contrasted with analog techniques, exemplified, for example, by a contacting potentiometer sensor and a non-contacting linear variable differential transformer (LVDT) sensor. Analog signal processing techniques are more sensitive to circuit component variations than are time-dependent signal processing techniques. Moreover, analog signal processing techniques require calibration and employ relatively large numbers of discrete circuit components. Consequently, time-dependent signal processing is favored.